This invention relates to wireless communication systems and, more particularly, to a method and apparatus for Doppler spread estimation in a wireless communication system.
In wireless mobile communication systems the transmission channel can vary with time. This is due to the mobility of the transmitter and/or the receiver. Variations in the transmission channel are characterized by the Doppler spectrum. The Doppler spectrum is the Fourier transform of the autocorrelation function of the channel""s time impulse response. The width of the Doppler spectrum is referred to as the Doppler spread in xe2x80x9cMicrowave Mobile Communications,xe2x80x9d W. C. Jakes, Jr., ed., New York: Wiley, 1974, IEEE Press, 1994, and xe2x80x9cWireless Communication,xe2x80x9d Theodore S. Rappaport, Prentice Hall PTR, 1996. The Doppler spread is commonly used as a measure of the channel variation rate or fading rate.
It is often desirable to estimate the Doppler spread. For example, in order to adaptively optimize a coherent detector in a receiver, the parameters of the adaptation algorithm are made to be a function of the estimated Doppler spread. Also, in a cellular mobile telephone system, the handoff process can be enhanced if an estimate of the Doppler spread is available. This avoids handoff of fast moving users to micro cells.
U.S. Pat. Nos. 4,723,303 and 5,016,017 discuss the use of Doppler spread estimation. Both patents describe estimating the Doppler spread from the received signal itself However, these methods may not give a good estimate with a non-stationary received signal. Lars Lindbom, xe2x80x9cAdaptive Equalization for Fading Mobile Radio Channels,xe2x80x9d Techn.Lic.Thesis No. UPTEC 92124R, November 1992, Department of Technology, Uppsala University, Uppsala, Sweden, discloses a method of estimating the Doppler spread from a sequence of channel estimates. This method uses differentials of the channel estimate, which comprise difference of values between two points in time. However, the differential itself is generally very noisy and needs averaging. As a result, the average gives biased estimates of the Doppler spread.
Still another proposal improves Doppler estimation by adaptive filters controlled by the final Doppler spread estimate. However, this is done with a significant increase of receiver complexity, while the estimate remains biased.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention there is provided a receiver Doppler spread estimator which utilizes stationarization of a non-stationary received signal.
Broadly, in accordance with one aspect of the invention, there is disclosed a Doppler spread estimator including receive means for receiving signals over a transmission channel that varies over time due to fading, and converting the signals to digital samples for processing. First processing means are operatively associated with the receive means for stationarizing the digital samples. Second processing means are operatively associated with the first processing means for estimating spectral densities of the stationarized digital samples. Third processing means are operatively coupled to the second processing means for correlating the spectral density estimates with functions corresponding to hypotheses of plural Doppler spread values to select one of the Doppler spread values as an estimate of actual Doppler spread.
It is a feature of the invention that the receive means is adapted to receive code division multiple access (CDMA) signals.
It is a further feature of the invention that channel coefficients are obtained directly from the CDMA signals.
It is another feature of the invention that the receive means is adapted to receive signals including pilot symbols spread with a known CDMA spreading code.
It is another feature of the invention that the received signal represents a sequence of transmitted symbols and the first processing means comprises a mixer for mixing the digital samples with a complex conjugate of the transmitted symbols. The complex conjugate of the transmitted symbols is derived from a channel estimator.
It is another feature of the invention that the first processing means comprises a low pass filter.
It is another feature of the invention that the second processing means develops vector data representing spectral components.
It is still another feature of the invention that the third processing means comprises a multi-channel correlator. The correlator calculates correlations between estimated spectral densities and weighting functions corresponding to the hypotheses of the plural Doppler spread values. The third processing means includes a selector for selecting the hypothesis producing a highest correlation value.
In accordance with another aspect of the invention, there is disclosed a receiving apparatus Doppler spread estimator including a receiver receiving signals over a transmission channel that varies over time due to fading and converting the signals to digital samples for processing. A stationarizer operatively associated with the receiver stationarizes the digital samples. An estimator is operatively associated with the stationarizer for estimating spectral density of the stationarized digital samples. The correlator is operatively coupled to the estimator for correlating the spectral density estimates with functions corresponding to hypotheses of plural Doppler spread values to select one of the Doppler spread values as an estimate of actual Doppler spread.
There is disclosed in accordance with a further aspect of the invention a method of estimating Doppler spread of a received signal comprising the steps. of receiving the signals over a transmission channel that varies over time due to fading, and converting the signals to digital samples for processing; stationarizing the digital samples; estimating spectral density of the stationarized digital samples; and correlating the spectral density estimates with functions corresponding to hypotheses of plural Doppler spread values to select one of the Doppler spread values as an estimate of actual Doppler spread.
More particularly, the present invention relates to a method and apparatus for Doppler spread estimation in a wireless communication system. The Doppler spread information can be used to optimize performance of a communication system. Applications include both base and mobile station receivers where a coherent detector is the preferred detector type.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.